In a typical legacy data processing system, firmware provides the machine instructions that control the system when the system is being powered up or has been reset, but before an operating system (OS) is booted. That is, the firmware controls the pre-OS or pre-boot operations. Firmware may also control certain operations after the OS has been loaded, such as operations for handling certain hardware events and/or system interrupts. The firmware may handle pre-boot and post-boot operations through a set of routines referred to collectively as a basic input/output system (BIOS). The BIOS thus provides the interface between the hardware components of the system and software components such as the OS.
For purposes of this disclosure, the term “firmware” refers to software that may execute in a processing system before the processing system has booted to an OS, software that may provide runtime services that allow the OS or other components to interact with the processing system hardware, and similar types of software components. Traditionally, firmware has typically been stored in non-volatile memory. In more recent years, however, processing systems have been developed that store firmware in other types of storage devices or obtain firmware from remote repositories.
For purposes of this disclosure, depending upon the particular implementation under consideration, the term “processing unit” may denote an individual central processing unit (CPU) within a processing system, a processing core within a CPU, a logical processing unit such as a hyper-thread (HT), or any similar processing resource, or any collection of such resources configured to operate collectively as a unit. In a system where multiple processing units exist, the operating system (OS) normally owns all of the processing units.
Also, in some processing systems, it is possible to hide one or more of the processing units from the OS, for instance by modifying the advanced configuration and power interface (ACPI) tables produced by the BIOS. In some systems it is also possible to hide one or more portions of random access memory (RAM) from the OS. Additionally, in some systems, several peripheral and integrated devices can be hidden from the OS, for example by updating device-hide registers or other locations in the system's input/output (I/O) controller hub (ICH). These techniques may be used to hide devices for debugging purposes.